Inductive Load Overcurrent Circuit Breaker Testing
In an AC circuit, current flows from the power source to the load, and passes through control devices such as switches and relays, or semiconductor solid-state relays for ON/OFF control. For current protection, devices such as fuses and circuit breakers are used. The current then reaches the load device.
This article introduces the use of the AEL series AC electronic load to simulate the current of load devices in the circuit (including both startup transients and steady-state operation) for testing and verifying components. In particular, the circuit includes components such as switches and current breakers when an inductive load is present, such as the above-mentioned switches, relays, fuses, breakers, etc.
In AC circuits, the types of loads include resistive loads, inductive loads, capacitive loads, and rectified loads. Resistive loads, such as heating devices, are common, while rectified loads are widely found in electronic products. These products typically use diodes for rectification, followed by filtering with capacitors and voltage conversion through transformers to provide the necessary DC power for the electronics. The rectifier circuit, which includes diodes and capacitors, is considered a rectified load. When the voltage is a 50/60 Hz sine wave, the current waveforms for resistive, inductive, and capacitive loads are also sine waveforms. However, for rectified loads, the current waveform is a pulsed waveform, roughly in phase with the voltage.
Inductive loads such as motors and compressors have a power factor lower than 1, and the current phase lags behind that of the voltage. If the power supply is suddenly interrupted or the current circuit breaker is activated during the operation of the inductive load, a back-EMF voltage may be generated on the inductive load. If the AEL AC electronic load is also connected in parallel to the inductive load, the back-EMF voltage on the inductor may exceed the rated voltage of the internal power components of the AEL AC electronic load and causes damage.
The back EMF voltage varies with the product of the L of the inductive load and di/dt, and di/dt varies with the sine wave angle of the AC power supply. Therefore, the back EMF voltage generated by power interruption at different sine wave angles will be different, usually 1 to 2 times the power supply voltage. When superimposed with the power supply voltage, it will be up to three times the power supply voltage.
When AEL is connected to an inductive load, a TVS must be added to limit the inductor's back EMF voltage.
The upper waveform in the figure below is the voltage of the inductor load, and the lower waveform is the inductive current. At about 42.5 ms, the inductive current is suddenly interrupted, resulting in a surge voltage on the inductive load.
Therefore, when the AEL Series AC electronic load is connected to an inductor or transformer, a TVS component must be added in parallel with the load input to limit the high voltage of L di/dt to prevent the internal MOSFET from exceeding the rated voltage and causing damage. As shown in the figure below, the back EMF voltage is limited by the TVS.
AC voltage = 200 V 50 Hz, inductive load = 100 mH, AEL = 500 Ω, Tvs = 30KPA288CA
The energy (joules) stored in the inductor is 1/2Li², which will generate electrical energy of transient power (Energy = power x time) in a short period of time. The back EMF voltage can usually be limited by voltage protection components, such as TVS or MOV to prevent the voltage from exceeding the maximum rated voltage of the electronic load and protect the MOSFET power components of the electronic load. The TVS also needs to have sufficient power specifications. The lower part of the above figure shows the energy consumption of TVS, with a power of 1.5 KW and a duration of about 2 ms.
Overseas Sales Department
Good Will Instrument Co., Ltd
No. 7-1, Jhongsing Road, Tucheng Dist.,
New Taipei City 23678, Taiwan R.O.C.
Email: marketing@goodwill.com.tw